Sex might seem like one of those little gifts from evolution. But it’s pretty inefficient from an evolutionary perspective. It’d be much easier to reproduce if you could do away with finding the right member of the opposite sex to help you create the next generation. So why did evolution come up with sex?

Biologists have hypothesized that one driving force might have been parasites. Now scientists have had a chance to test that theory. Asexual reproduction leads to clones. Being genetically identical, clones are also weak in the same ways, and thus more likely to all succumb to a parasite. But sex keeps shuffling the genetic deck.

Well, there’s a snail common in New Zealand lakes that does both—some populations have sex and some reproduce asexually. So researchers spent 10 years monitoring the two populations, and the number of parasites living off both groups. As expected, cloned snails that were plentiful at the beginning of the study suffered big losses as they became infected with parasites. But the sexual snail populations remained stable, results published in the journal American Naturalist. So, next time you’re feeling sexy, thank a parasite.

—Cynthia Graber